668 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



of tliis view is based on types that have a world-wide distribution and on those that are only 

 similar, not specifically identical. In my opinion direct connection has not been proved. In 

 time range the Chico formation apparently began somewhat earlier and continued somewhat later 

 than the Colorado fauna of the interior seas, but it did not extend to the end of the Cretaceous, 

 and latest Cretaceous time is probably not represented by marine deposits on the Pacific coast. 



The character and relations of the Chico vary in different locahties from south- 

 ern California northward through the Coast Range and the northern Sacramento 

 Valley. The Chico in the San Luis quadrangle is described by Fairbanks ^^* as 

 follows : 



The Atascadero formation, the local representative of the Chico group, consists mainly 

 of certain soft sandstones of a grayish-yellow color. The formation derives its name from 

 Atascadero Creek. A few poorly preserved fossils were found in it at several points along the 

 northern slope of the Santa Lucia Range. 



Near the coast the Atascadero formation occurs above the San Luis (pre-Cretaceous) 

 formation, but northeast of the Santa Lucia Range it rests on the Toro Qower Cretaceous). 

 In the latter region the Atascadero terminates downward in a conglomerate which is in places 

 100 feet thick and contains large granite bowlders. The striking contrast in general lithologic 

 character between the Atascadero and Toro formations is indicative of a marked change in 

 conditions of deposition. The hj^othesis that there is a hiatus in the Cretaceous sediments is 

 well founded. It is based, on the one hand, on the fact that the Atascadero sediments extend 

 over the Toro across both strike and dip, indicating that the Toro had been upturned and planed 

 off before the Atascadero began to be deposited (as may be seen at many points, particularly 

 on the divide between Atascadero and Santa Margarita creeks), and, on the other hand, on the 

 fact that the Atascadero in the same locality rests indiscriminately upon the Toro and San 

 Luis formations. 



Another interesting fact should be mentioned in connection with the discussion of an uncon- 

 formity. The serpentine here, as in other portions of the Coast Ranges, wherever it comes in 

 contact with the lower Cretaceous is intrusive, while it has nowhere been observed to penetrate 

 the Atascadero formation. There were at least two epochs of igneous activity during the 

 Cretaceous, and three if the formation of the San Luis Buttes be included; this supports the 

 view that marked movements occurred during Cretaceous time. 



The Chico is developed on the east side of the Coast Range in the mountains 

 bordering the San Joaquin Valley, according to Arnold and Anderson.^*'' A quo- 

 tation from their paper has been given in Chapter XIV (pp. 616-617), outlining the 

 general sequence of the Cretaceous beds in the Coalinga district. The rocks are 

 there separable into three divisions, of which the lowest is supposedly of KnoxviUe 

 (Lower Cretaceous) age, and the middle and uppermost contain Chico (Upper 

 Cretaceous) fossils. The Chico portion is described as follows : 



The middle one of the main divisions into which the Knoxville-Chico rocks are divisible upon 

 lithologic grounds comprises a thick series of alternating thin beds of dark shale and sandstone 

 with the above-mentioned heavy conglomerate at its base and with the characteristic massive, 

 concretionary sandstone beds that form the upper division overlying. Its thickness, including 

 the conglomerate zone, measures at least 4,800 feet. * * * The conglomerate is locally 

 extremely variable and is not continuous. It may in places be traced directly into massive 

 sandstone or even into thinly bedded sandstone and shale, as along Alcalde Canyon between 

 2 and 3 miles west of Alcalde. It represents throughout the district, however, an important 

 stratigraphic horizon characterized by a coarsening of the sediments. * * * 



The uppermost of the three divisions of the Kn ox ville-Chico is predominantly concre- 

 tionary sandstone in the lower part and shale in the upper part and has a thickness of at least 

 4,700 feet. * * * Taken as a whole the upper division is strikingly distinct from other 



